Lesson 4.8DeformChapter 4 · Sheet and tube forming

Electromagnetic forming

A magnetic pulse throws metal onto a fitting, without contact

Electromagnetic forming (EMF) shapes metal with a pulse of magnetism instead of a tool that touches it. A coil fed with a split-second burst of current repels an aluminum or copper part, hard enough to crush a tube onto a fitting. The same physics lets hobbyists shrink a coin, and lets carmakers join aluminum tubes to steel parts without welding.

Photo: Bert Hickman, Stoneridge Engineering, CC BY-SA 3.0, via Wikimedia Commons

electromagnetic crimping · pulse circuit and cut-away of coil, tube and fittingCircled numbers match the steps below. The cut-away shows only the upper half of the round coil, tube and fitting, just after the pulse.

How it works

Charge the capacitors. A capacitor bank (a fast-releasing store of electric charge) is charged to several kilovolts, typically storing a few to tens of kilojoules.
Fire the switch. A fast switch (a spark gap or a high-power thyristor) connects the bank to the work coil, a few turns of heavy, well-insulated copper.
Send a current pulse. Tens to hundreds of thousands of amps rush through the coil for about 10–100 microseconds. The sudden magnetic field induces eddy currents (swirling currents induced in a conductor) in the tube, flowing the opposite way.
Let the push do the work. Opposite currents repel each other, so a strong magnetic pressure throws the tube wall away from the coil at roughly 100–300 m/s. The coil never touches the part.
Impact and lock. The wall slams onto the fitting and into its grooves, making a tight joint in microseconds. With enough speed the two metals can even weld (magnetic pulse welding).

Key facts

Materials: Good electrical conductors: aluminum, copper, brass; steel and poor conductors need adaptations or a conductive "driver" sheet
Typical parts: Tubes crimped onto fittings, cable lugs, drive shafts with end fittings, sheet details pressed into a one-sided die
Part size: Mostly tubes and sheet areas up to a few hundred millimeters, set by the coil and the stored energy
Precision: Very repeatable; the shape is set by the fitting or die, often within about ±0.1 mm on crimp diameters
Surface finish: No tool marks from the coil; the formed face copies the fitting or die
Batch size it suits: Medium to high series once coil and settings are tuned
Tooling and machine cost: Pulse machines typically cost hundreds of thousands of dollars; coils wear out and are replaced
Speed: Forming takes microseconds; with loading, a cycle takes a few seconds

Strengths and limits

Strengths

No contact: no tool marks and no lubricant
Extremely fast and very repeatable
Joins dissimilar materials, such as aluminum to steel, without heat
High speed lets metal stretch further, with little springback

Limits

Works best on good conductors such as aluminum and copper
High voltage demands strict electrical safety
Coils suffer huge forces and wear out
Limited to fairly thin walls and modest part sizes

Where it's used

AutomotiveDrive shafts, frame joints, air-conditioning lines
AerospaceTorque tubes and control rods joined to end fittings
ElectricalCrimped cable lugs and connectors
Heating and coolingCopper-to-aluminum refrigerant tube joints

How to recognize it

A tube shrunk smoothly and evenly into the grooves of a fitting, all the way around.
No tool marks, dents or jaw prints where the crimp is.
Often aluminum or copper joined to steel or plastic with no weld bead or adhesive.

Compare with

Explosive forming4.9also high-speed forming, but the energy comes from a chemical charge under water, for very large parts.Hydroforming4.5pushes metal with liquid pressure, slowly and with seals; EMF uses a magnetic pulse and no contact.Explosion welding7.8the same "fast collision bonds metals" idea, driven by explosives.

Watch it

Physics Girl · A clear explanation, with slow-motion footage of a coin shrunk by a pulsed coil

Physics Girl · How violent the magnetic push is, seen in slow motion

Chaotic Good · The hardware: capacitors, switch and coil of a pulse machine

Greg Benoit · An early aerospace use: tubes joined to end fittings by EMF

See real photos

Wikipedia: Electromagnetic forming — principle, materials, crimping and welding; almost no photos
Wikimedia Commons photo search

Remember

A current pulse in a coil pushes away a nearby conductor: no tool ever touches the part.
The whole forming happens in microseconds, at speeds of hundreds of meters per second.
Best for aluminum and copper tubes crimped onto fittings, even of a different metal.